As an optical amplifier apparatus used for an optical sea bottom transmission system, etc., whose transmission distance is long, it has been proposed that a remote pumping type optical amplifier apparatus is used. The remote pumping type optical amplifier apparatus has an advantage in that a pumping light source may be installed on land without being submerged on a sea bottom. However, since, in the remote pumping type optical amplifier apparatus, power of pumping light is attenuated by a transmission loss of optical fibers between a pumping light source and an erbium-doped optical fiber, it is necessary to increase output of the pumping light source. As one of the means for increasing the output of the pumping light source, such a method has been proposed, in which a plurality of pumping light having wavelengths different from each other are multiplexed as regards the wavelengths.
On one hand, in a wavelength division multiplexed transmission system, an attempt has been made, in which a degree of multiplexing is increased to increase the amount of transmission. Although it is considered, as one of the means for increasing the degree of multiplexing, that a wavelength band of optical fibers is widened, general erbium-doped optical fibers have only a gain band of 30 nm at maximum, and even though the band is widened to a longer wavelength band by lengthening the length of optical fibers, the erbium-doped optical fibers have only a gain of 60 nm at maximum. Therefore, based on the above description, Raman amplification, by which the gain band can be infinitely expanded, is now greatly expected. But, in order to infinitely widen the band in the Raman amplification, it is necessary to prepare a plurality of pumping lights of wavelength different from each other in order to infinitely widen the band in the Raman amplification, and wavelength multiplexing of pumping lights is requisite.
In various optical amplifiers as described above, it is necessary to multiplex pumping light sources having a plurality of wavelengths at a low loss. In cases where the number of pumping wavelengths is small as in prior arts, for example, although a dielectric multi-layered film filter is used as a multiplexing means in a case where the number of wavelengths is three or so, there was a shortcoming where the multiplexing loss is increased in line with an increase in the number of wavelengths in which the number of wavelengths becomes four or more.
Therefore, recently, it is proposed that a Mach-Zehnder type multiplexer is used as one of the methods for increasing the number of wavelengths with the multiplexing loss reduced. With a Mach-Zehnder type multiplexer, it is possible to increase the multiplexing loss on the basis of this principle even though the number of wavelengths is increased.